SIMBAD references

We derived position-velocity density distribution diagrams along the major (PA = 77°) axis of the elliptical planetary nebula NGC 7009 with the Keck HIRES [S II] 6716/6731 Å doublet spectral images. The average densities of the main shell and knots of NGC 7009 derived from the [S II] 6716/6731 Å fluxes integrated over the line of sight indicate a density range of N_e = 10^3.4 to 10^3.9 cm^–3, while the local densities from the volume fraction resolved in radial velocities along the line of sight show a considerably large range of about 10^2.8-10^4.7 cm^–3. The derived projection angle of the major axis of the main shell is ψ ∼ 18.3° (±2°). Assuming that the main shell is an ellipsoidal shell with a ≃ 16″ and b ≃ 6″, we found the range of the expansion velocity, radius, and latitude of four knots and a hot bubble. The four knots at points in symmetrical positions are roughly aligned with the same axis of expansion of latitudes ϕ ∼ ±34.5° (±0.6°): one pair expands at about 35 km s^–1 close to the main ellipsoidal shell, and the other expands rapidly at about 60 km s^–1 at a distance of r ∼ 16″. In the latitude range ϕ = 65°-75°, the hot bubble of a relatively large structure expands rapidly with a velocity of 130-150 km s^–1. The four knots and hot bubble points, which expand faster than the main shell, appear to have been formed by two to three eruptions at a different epoch from the primary structure formation.